Mass spectrometers have been used to analyse a wide range of materials, including organic substances such as pharmaceutical compounds, environmental compounds and biomolecules. They are particularly useful, for example, for DNA and protein sequencing. In such applications, there is an ever increasing desire for high mass accuracy, as well as high resolution of analysis of sample ions by the mass spectrometer, notwithstanding the short time frame of modem separation techniques such as gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS) and so forth.
One of the new directions in the field of mass spectrometry is the development of mass analysers where ions are dynamically trapped in an electrostatic field. Broadly, these may be divided into two classes: those that employ frequency analysis by image current detection, as disclosed in U.S. Pat. No. 5,880,466 and U.S. Pat. No. 5,886,346, and those that employ time of flight (TOF) separation and ion detection by secondary electron conversion, as is disclosed, for example by H. Wollnik, in J. Mass Spectrom. Ion Proc. (1994), vol. 131, at pages 387–407, and by C. Piadyasa et al., in Rapid Commun. Mass Spectrom. (1999), vol. 13 at pages 620–624. Although the trap fields may be ramped at the beginning of the mass scan, they are typically held very stable during the detection, or TOF separation of ions, and so each of the foregoing mass analysers may be regarded as electrostatic traps (ESTs).
Such EST mass analysers can achieve high and even ultra-high mass resolutions (in excess of 100,000), thus allowing more accurate determination of ion masses. However, they all operate using an inherently pulsed technique and as such the task of coupling to any external continuous ion source is a serious problem.
To improve duty cycle and sensitivity, it is possible to use an external collision quadrupole ion trap for ion cooling and storage between injections. This technique has proved particularly successful when combined with other inherently pulsed techniques such as TOF mass analysis as is described by S. Michael et al., in Rev. Sci. Instrum. (1992) vol. 63, pages 4277 to 4284. Here, ions are accumulated in the trap. As suggested in U.S. Pat. No. 5,572,022, it is possible to control the number of ions in the trap to reduce space-charge effects. Once ions have been stored in the trap, they can be pulsed into the TOF mass analyser by applying high voltages to the (normally grounded) end caps of the trap. In U.S. Pat. No. 5,569,917, the ions are given a simultaneous “push” out of the trap and a “suck” from the TOF mass analyser, so as to improve the efficiency of ion injection into the analyser. The spatially spread ion beam is focussed into a tight pack in the “object” plane of the TOF mass analyser.
Despite these improvements, quadrupole ion traps are still currently a relatively inefficient technique for injecting ions into a mass analyser (down to a few percent), and they also suffer from low space charge capacity due to the limited trap volume.
One approach that has been taken to address these problems is to employ a different type of collisonal storage device known as a linear trap (LT) or RF multipole trap. U.S. Pat. No. 5,179,278 shows such an arrangement, wherein a two-dimensional multipole RF field is generated. The trap of U.S. Pat. No. 5,179,278 is limited by end lenses. Alternatively, the poles of the trap may be split into sections as is shown in U.S. Pat. No. 5,420,425. Both split poles and end caps can be employed together. The elevated voltages on the end lenses or sections limit the ion movement along the axis whilst the RF voltage provides a quasi-potential well in the radial direction. If ions lose enough energy during the first passes through the multipole, then they may be trapped in it and squeezed towards the axis during further collisions. The number of ions in the trap can be controlled using a short pre-scan, a technique disclosed in the above-referenced U.S. Pat. No. 5,572,022. Nevertheless, to inject ions from the LT into the next stage of analysis, the voltage is lowered on the exit lens and the ions in the LT are allowed to flow out of the multipole. This flow typically lasts up to hundreds or even thousands of microseconds. These time scales are compatible with the injection times for quadrupole ion traps (as disclosed in the above-referenced U.S. Pat. No. 5,179,278) or for Fourier Transform Ion Cyclotron Resonance (FTICR) as set out by M. Senko et al in JASMS, (1997), volume 8, pages 970–976. The time scales are also suitable for orthogonal acceleration TOF mass spectrometry, see for example U.S. Pat. No. 6,011,259, U.S. Pat. No. 6,020,586, and WO99/30350.
Segmented construction of the poles in the LT may be employed, as set out by M. Belov et al, in Analytical Chemistry (2001) volume 73, pages 253–261, to reduce the injection time down to about 300–400 microseconds. The segmented construction of the LT provides an axial field which causes ions to be displaced towards the exit lens.
Even so, such injection times are too long for an electrostatic trap. This is because ESTs require high ion energies (typically 1–2 keV per charge) to achieve dynamic trapping. If injection takes place over hundreds of microseconds, at such energies the process may last for hundreds of ion reflections. Without any collisional cooling inside the electrostatic trap, ion stability may be compromised.